chore: add Makefile and golangci-lint configuration

## Problem The project has no Makefile or linter config. CI runs `go vet` but no linter. A Makefile with standard targets makes contributor onboarding easier and is expected for open-source Go projects. ## Proposed fix Add a `Makefile` with common targets: - `make build` — build the binary - `make test` — run tests - `make vet` — run go vet - `make lint` — run golangci-lint - `make fmt` — run gofmt - `make install` — install to GOPATH/bin Add a `.golangci.yml` with sensible defaults (errcheck, govet, staticcheck, ineffassign, unused). ## Acceptance criteria - [ ] Makefile with all targets above - [ ] `.golangci.yml` with default linters enabled - [ ] `make lint` passes with zero warnings on current codebase - [ ] CI workflow updated to run `make lint`